The present invention relates to a photometry device applicable to an SLR (Single Lens Reflex) camera, and more particularly to a photometry device with which exposure errors due to a difference of reflectivity of objects having different colors can be compensated.
Recently, reflection type photometry devices have been employed in most of cameras. The reflection type photometry device receives light, which is reflected by an object and passed through an observing optical system of a camera, using a light receiving element, determines the brightness of the object based on the output of the measured value, and then calculates the exposure value of the camera based on the measured brightness.
However, this type of the photometry device cannot detect the color of the object because of its structure. Accordingly, in such a device, the reflectivity of an object is generally assumed to be 18% and the exposure parameter is determined on this assumption. Therefore, regarding a whitish object whose reflectivity is greater than 18%, the determined brightness is greater than the actual brightness because the determined brightness assumes only 18% reflectivity contributing to the brightness, when the reflectivity contribution is actually higher. If the camera controls an exposure operation based on the determined exposure value, the object is under exposed. A dark object whose reflectivity is less than 18% is measured to have a lower brightness than the actual brightness because the measured brightness assumes 18% reflectivity contributing to the brightness, when the reflectivity is actually lower. Therefore, such an object is over exposed. The difference of the reflectivity of the object may also occur depending on the color cf the object. For example, when the color of an object is yellow, the reflectivity may be up to 70%. In such a case, if the standard reflectivity is assumed to be 18%, the exposure value is approximately 2 Ev lower than necessary. If the object color is blue, the reflectivity is approximately 9%. In this case, the object is over exposed by approximately 1 Ev greater than necessary.
Therefore, when the conventional photometry device is used, the photographer is required to guess the reflectivity of the object. Then, based on the reflectivity determined by the photographer, the exposure is controlled such that, if the object is a whitish or yellowish one having a relatively high reflectivity, it is to be overexposed, and if the object is a blackish or bluish one having a relatively low reflectivity, it is to be underexposed. With this operation, the above-described defects may be solved. However, accurately guessing the reflectivity of the object and controlling the exposure can only be done by experienced and skilled photographers. It is impossible to require all the photographers to do such an operation. Further, it is not preferable that a manual operation of the photographer is required for exposure. Furthermore, if such a manual operation is required, cameras become unsuitable for automatic photographing which is the recent trend.
It is therefore an object of the invention to provide an improved photometry device which is configured to detect photometry compensation values at a plurality of divided areas of an object, and applies colorimetric compensation by weighting the compensation values depending on the divided areas.
For the above object, according to the invention, there is provided a photometry device for a camera, which is provided with a normal light sensor that has a plurality of photometry areas and performs a photometry operation with respect to an object at each of the plurality of photometry areas, the normal light sensor having a spectral sensitivity characteristics close to those of a human eye, a plurality of photometric sensors for colorimetry capable of performing photometry with respect to each of the plurality of photometry areas, the plurality of photometric sensors having different spectral sensitivity characteristics, a photometry value determining system that determines an photometry value at each of the plurality of photometry areas in accordance with outputs of the normal light sensor corresponding to the plurality of photometry areas, a colorimetric compensation value determining system that determines a color of an object at each of the plurality of photometry areas in accordance with the outputs of the plurality of photometric sensors for colorimetry and determines a colorimetric compensation value based on the determined color, and an exposure value determining system that compensates for the photometry value determined by the photometry value determining system for each of the plurality of photometry areas, and determines an exposure value based on the compensated photometry values. With this configuration, the colorimetric compensation value determining system determines different threshold values, which are used for determining colors at the plurality of photometry areas, for different ones of the plurality of photometry areas.
With this configuration, it becomes possible to determine appropriate colorimetric compensation values depending on the arrangement of the areas within a photographing frame.
Optionally, the colorimetric compensation value determining system may determine the threshold values such that a threshold value at a peripheral area of a photographing frame is greater than that of a central area of the photographing frame.
Further optionally, the colorimetric compensation value determining system may determine the threshold values for the plurality of photometry areas depending on a distance of each photometry area with respect to the center of a photographing frame.
In this case, a threshold value at an area farther from the center of the photographing frame is greater than that at an area closer to the center of the photographing frame.
Still optionally, the colorimetric compensation value determining system may determine the threshold values for the plurality of photometry areas depending on data intrinsic to a photographing lens that forms an image of the object.
In this case, the data intrinsic to the photographing lens may include at least one of a focal length of the photographing lens, an exit pupil position of the photographing lens, and an open f-number of the photographing lens.
Alternatively, the data intrinsic to the photographing lens may include a focal length of the photographing lens, and wherein a threshold value has a smallest absolute value when the focal length is within a predetermined range, the threshold value being greater when the focal length is greater or smaller than the predetermined range.
In this case, the threshold value may be determined as a function of the focal length.
Further alternatively, the data intrinsic to the photographing lens may include an exit pupil position of the photographing lens, and a threshold value may have a greatest absolute value when the exit pupil position is within a predetermined range, the threshold value being smaller when the exit pupil position is on a front side or rear side with respect to the predetermined range.
In this case, the threshold value may be determined as a function of the exit pupil position.
Further alternatively, the data intrinsic to the photographing lens may include an open f-number of the photographing lens, and a threshold value may have a greatest absolute value when the open f-number is within a predetermined range, the threshold value being smaller when the open f-number is smaller or greater than the predetermined range.
In this case, the threshold value may be determined as a function of the open f-number.
Optionally, the colorimetric compensation value determining system may determine the threshold values for the plurality of photometry areas depending on an area corresponding to a part of an object on which a photographing lens is focused.
In this case, a threshold value for an area corresponding to a part of the object on which the photographing lens is focused may be greater than that for an area corresponding to another part of the object on which the photographing lens is not focused.
Further optionally, t:he exposure amount determining system has at least one of: (a) function of determining the exposure amount by performing a divided photometry, based on the compensated photometry values, in accordance with a predetermined algorithm, (b) a function of determining the exposure amount by averaging the compensated photometry values for the plurality of photometry areas, (c) a function of determining the exposure amount by performing the center-weighted averaging of the compensated photometry values, and (d) a function of determining the exposure amount by selecting one of the compensated photometry values corresponding to the plurality of areas.
Further optionally, the normal light photometry sensor and the plurality of photometric sensors for colorimetry are arranged on an eyepiece side of a pentagonal prism of a single lens reflex camera, at least the normal light photometry sensor being arranged at an upper central portion of the pentagonal prism.
Furthermore, the plurality of photometry sensors for colorimetry may include a green light photometry sensor for detecting green light, the green light photometry sensor functioning also as the normal light photometry sensor.
Still optionally, the colorimetric compensation value determining system may determine the colorimetric compensation value by calculation.